Ag Alloying in Cu2-yAgyBa(Ge,Sn)Se4 Films and Photovoltaic Devices

Trigonal Cu2BaGe1-xSnxSe4 (CBGTSe) has recently gained interest as a potential photovoltaic absorber to target mitigation of antisite defect formation in Cu2ZnSn(S,Se)(4). This study examines partial substitution of Cu by Ag as a potential approach to tune the properties of Ag-incorporated CBGTSe in the following aspects: 1) phase stability and crystal structure as a function of Ag content; 2) film morphology and grain structure; 3) charge carrier properties; 4) band positions; and 5) charge carrier kinetics and recombination. Up to 20% of Cu can be substituted by Ag in CBGTSe, while above 20% a phase mixture appears. Increasing Ag content induces larger average grain size and reduced hole carrier densities. In contrast, photoelectron spectroscopy and photoluminescence measurements reveal negligible impact of Ag substitution on ionization potential (approximate to 5.4 eV) and electron affinity (approximate to 3.7 eV). Also, Ag content offers negligible impact on carrier lifetimes (few ns). Consistent with these fundamental properties, solar cells based on two different Ag/(Ag + Cu) ratios (approximate to 0% and approximate to 20%) show comparable power conversion efficiencies (approximate to 2.7-2.8%). These results indicate that CBGTSe films and solar cells may be less sensitive to Ag substitution compared to Cu2ZnSn(S,Se)(4), at least at the current level of absorber and device optimization.

Automated summary

This study investigates the effects of Ag substitution on the properties of Ag-incorporated CBGTSe, including phase stability, crystal structure, film morphology, grain structure, charge carrier properties, band positions, and charge carrier kinetics and recombination. It is found that up to 20% of Cu can be substituted by Ag in CBGTSe, and increasing Ag content leads to larger grain size and reduced hole carrier densities. However, Ag substitution has negligible impact on ionization potential, electron affinity, and carrier lifetimes. Solar cells based on different Ag/(Ag + Cu) ratios show comparable power conversion efficiencies.